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Journey through nearly 14 billion years of history with the Big History Project, now available on Khan Academy. This course asks the big questions about our Universe, our planet, life and humanity. Examine our shared history across scales and disciplines. From the Big Bang to our still expanding universe, this course, created and maintained by the Big History Project, will lead you on a journey of astounding connections and exciting discoveries.

Early humans had pretty small social networks. At most, they probably met only a couple hundred people who probably all lived very similar lives to their own. As people started farming, these networks got larger. People were increasingly specialized in their work and trade. Populations in cities got larger. Trade reached across longer distances, bringing together people with very different lives and ways of thinking.

For most of the agrarian era, the world was divided into four separate and distinct world zones. Over time, these zones slowly became more connected as networks of communication and exchange expanded. While innovations did occur throughout this era, such as irrigation, iron plows, and fast-ripening rice, none of these innovations were able to sustain long-term population growth, which limited expansion. Each innovation led to immediate growth, but once populations had grown beyond a certain point, they fell. These cycles of rise and fall in population, called Malthusian cycles, characterized the agrarian era. Humans would not break out of these cycles until the world zones became more connected and rates of innovation were capable of sustaining growth over much longer periods of time.

The rise of agriculture ushered in an era of increasing innovation in communication and transportation that led different parts of the world to connect in entirely new ways. The voyages of Christopher Columbus extended this exchange from Afro-Eurasia to the Americas, which saw a massive movement of ideas, people, diseases, plants, and animals between the two hemispheres. The results of these exchanges were dramatic. Potatoes and corn, first cultivated in the Americas, quickly became crucial in the diets of people across Eurasia. Horses and cattle, unknown in the New World in 1492, quickly took on crucial roles in many societies in the Americas. The linking of the different world zones in this period and the exchanges that this linking made possible, transformed the lifeways of the people and civilizations involved – and laid the foundation for modern exchange routes and the global balance of power.

Evading bandits through mountain passes, leading a caravan of yaks carrying silk and goods, sailing the trade winds off the Indian coastline – these are a few things you might have done as a trader in the age of agrarian civilizations. Systems of exchange and trade between large agrarian civilizations facilitated the transfer of goods from one civilization to the next, but they also helped share the world’s religions, ideas, innovations, diseases, and people. While each world zone had its own trade routes, none were as vast and intense as the Silk Road. This large system of exchange and trade, initially designed for commerce, dispersed goods and ideas throughout Afro-Eurasia, and paved the way for a substantial increase in both commerce and collective learning.